BASIC is a good language to start on. Good luck!
As for Fief: I added mechanics for when the character is busy on a task (such as chopping down a tree), such that time flies by unless they are interrupted or you cancel the action. I added some algorithms for skill level bonuses and bonuses from attributes (such as Strength and Endurance for tree choppin'), also the effect of the quality of the tool plays a role as well. All of these factor into the amount of time it takes to chop down the tree and (eventually) it will determine the quality of the wood produced.
The way I set the skills is such that it takes about 5000 hours of work to become an expert level in any skill. I was basing this off the idea that it takes 10,000 hours to become an expert at something. This seemed way too long for most of these tasks, so I halved it. This is still 208 consecutive days of experience, and everyone needs to eat and sleep. I have things set so that you'll gain skill levels quicker at lower levels, but require more and more time as you increase. Skill levels range from 0 (Unskilled) to 10 (Expert), although the actual modified time (stored as a floating point) is used such that every little bit of experience helps (down to the tiniest fraction). Each 1000 points reflects a new skill level. In this way, progression will be very realistic, so for us used to DF, we'll find this to be tediously slow. But this also makes that Lumberjack peasant who's been chopping trees for the past 20 years someone worth protecting.
I spent last night looking at the effects of Skill, Attribute, and Tool bonuses on the average time to chop down a mature tree. I'll post those here later for the number geeks.
For the curious, the attributes for the game will be posted later as well. This is taken from my pen & paper RPG I've been working on for a number of years, and will also make an appearance in Endless Worlds if I ever get around to working on that game again. It basically comes from the idea that there are three components to a person: Body, Mind, and Spirit, being the physical, mental, and spiritual sides respectively. By squaring these components off, you get an attribute for when each component governs another or governs itself.
Peasants and other common folk probably will not have attributes for space-saving reasons. They will rely upon Skills and Tools alone for improvements. Attributes are more of a Hero/Villian aspect. Their actual game effects are small compared to Skills (at least in most Fief tasks), but will give your character a slight edge. They will play a much greater role in combat and diplomacy, as other Nobles, Knights, Villains, Priests, and other "named" characters will have them. So when fighting a regular soldier, those bonuses will give you an edge, but when you face their leader in a duel, you'll find it a much more equal challenge.